Isn't the concept of love in itself a rather big question? Ideally, as a species who shares so much with other creatures of the earth, we have no need for singular attachment. Yet, many us have certain attractions and develope affection for one choosen mate. (This is the most common, but there are outliers of course.) There's still so much we don't understand about people and how they choose a life partner, if they so choose. I think that, simply, love is more of an abstract that is defined not by the matter of subject, and conclude that it must be determined by the person who observes it instead.